drawings were made from photographs

drawings were made from photographs printed at ´ 100 and ´ 500 magnifications. Measurements
included: (1) total length of the prosicula, measured along the axis from base of nema to prosicularmetasicular
contact [PL]; (2) width of prosicula at contact with nema [NW]; (3) width of prosicula at
prosicular-metasicular contact [BW] and (4) length of prosicula measured along margin [PC]. The plot of
total length vs width (PL:BW; Text-fig. 3) indicated a degree of clustering that might permit taxonomic
differentiation in the future once greater numbers of specifically identifiable specimens had been utilized.
It was expected that the ratio PL:PC would give an indication of the relative curvature of the prosicular
margin, larger ratios indicating a more rounded margin. The two values were, however, so close in most
taxa that no clear pattern emerged; this was also the case in the width of the prosicula at the nemal contact
(NW).
EVOLUTIONARY TRENDS
In addition to small-scale differences between the prosiculae of taxa studied, it became apparent that
broader, general morphological modifications had occurred during the early to mid Ordovician (early
Tremadoc through Llanvirn). Changes occurred in the position (and presence?) of the diaphragm, shape
and size ratio of the conus and cauda, prominence of the spiral line and nature of the longitudinal ridges
(Text-fig. 4). The earliest graptoloid siculae from the early Tremadoc anisograptids (e.g. Rhabdinopora
and Staurograptus; see Cooper et al. 1998) possess long prosiculae which pass imperceptibly through the
cauda into the nema and exhibit a strong, widely spaced spiral line with very low angle of inclination
distally, making the prosicula-metasicula contact difficult to distinguish (Pl. 1, figs 1–3). All show a
prominent diaphragm which divides the prosicula into a conus and cauda of similar length. Notably
lacking is any sign of longitudinal ridges; in fact, the only longitudinal lines are present along the nema.
No siculae of mid Tremadoc age were examined during the present study; the prosiculae of
Adelograptus, described from this interval by Hutt (1974), appear, however, to be very similar to our
early Tremadoc forms. Late Tremadoc prosiculae, belonging to genera such as Kiaerograptus and
Aorograptus (see Williams and Stevens 1991) have a more ‘advanced’ appearance in terms of the overall
form, which is typically an elongate ‘bell-shape’ with a highly constricted neck where the diaphragm
1008 PALAEONTOLOGY, VOLUME 42
TEXT-FIG. 3. Bimodal plots of prosicula length versus prosicula apertural width.

النتائج (العربية) 1:

وقدمت الرسومات من الصور الفوتوغرافية المطبوعة في ´ 100 وتكبير 500 ´. القياساتوشملت: (1) يبلغ طول بروسيكولا، يقاس على طول المحور من قاعدة نعمة على بروسيكولارميتاسيكولارالاتصال [رر]؛ (2) عرض بروسيكولا في اتصال مع نعمة [شمال]؛ (3) عرض بروسيكولا فيبروسيكولار-ميتاسيكولار الاتصال [BW] وطول بروسيكولا (4) قياس طول الهامش [الكمبيوتر]. المؤامرة منالطول الإجمالي مقابل العرض (PL:BW؛ Text-fig. 3) أشارت إلى درجة من المجموعات التي قد تسمح التصنيفيةالتمايز في المستقبل متى تم استخدام عدد أكبر من العينات يمكن تحديدها على وجه التحديد.وكان من المتوقع أن نسبة PL:PC أن تعطي مؤشرا لانحناء النسبي بروسيكولارالهامش، ونسب أكبر مما يشير إلى هامش أكثر تقريب. هذه القيم غير، قريبة جداً في معظمالأصناف التي ظهرت أي نمط واضح؛ وكان هذا أيضا الحالة في العرض بروسيكولا في الاتصال نيمال(شمال غرب).الاتجاهات التطوريةوبالإضافة إلى صغار الاختلافات بين بروسيكولاي الأصناف التي درست، أصبح من الواضح أنالتعديلات الشكلية العامة الأوسع نطاقا، وقد حدث خلال وقت مبكر إلى منتصف Ordovician (أوائلتريمادوك من خلال لانفيرن). حدثت تغييرات في موقف (والوجود؟) الحجاب الحاجز، الشكلونسبة الحجم كونوس وكودا، وبروز خط دوامة وطبيعة التلال طولية(نص الشكل 4). سكولا جرابتولويد أقرب من أنيسوجرابتيدس تريمادوك المبكر (مثلاً رهابدينوبوراand Staurograptus; see Cooper et al. 1998) possess long prosiculae which pass imperceptibly through thecauda into the nema and exhibit a strong, widely spaced spiral line with very low angle of inclinationdistally, making the prosicula-metasicula contact difficult to distinguish (Pl. 1, figs 1–3). All show aprominent diaphragm which divides the prosicula into a conus and cauda of similar length. Notablylacking is any sign of longitudinal ridges; in fact, the only longitudinal lines are present along the nema.No siculae of mid Tremadoc age were examined during the present study; the prosiculae ofAdelograptus, described from this interval by Hutt (1974), appear, however, to be very similar to ourearly Tremadoc forms. Late Tremadoc prosiculae, belonging to genera such as Kiaerograptus andAorograptus (see Williams and Stevens 1991) have a more ‘advanced’ appearance in terms of the overallform, which is typically an elongate ‘bell-shape’ with a highly constricted neck where the diaphragm1008 PALAEONTOLOGY, VOLUME 42TEXT-FIG. 3. Bimodal plots of prosicula length versus prosicula apertural width.